35.8: How Neurons Communicate - Synaptic Plasticity
- Distinguish between long-term potentiation and long-term depression
Synaptic plasticity is the strengthening or weakening of synapses over time in response to increases or decreases in their activity. Plastic change also results from the alteration of the number of receptors located on a synapse. Synaptic plasticity is the basis of learning and memory, enabling a flexible, functioning nervous system. Synaptic plasticity can be either short-term (synaptic enhancement or synaptic depression) or long-term. Two processes in particular, long-term potentiation (LTP) and long-term depression (LTD), are important forms of synaptic plasticity that occur in synapses in the hippocampus: a brain region involved in storing memories.
Short-term Synaptic Enhancement and Depression
Short-term synaptic plasticity acts on a timescale of tens of milliseconds to a few minutes. Short-term synaptic enhancement results from more synaptic terminals releasing transmitters in response to presynaptic action potentials. Synapses will strengthen for a short time because of either an increase in size of the readily- releasable pool of packaged transmitter or an increase in the amount of packaged transmitter released in response to each action potential. Depletion of these readily-releasable vesicles causes synaptic fatigue. Short-term synaptic depression can also arise from post-synaptic processes and from feedback activation of presynaptic receptors.
Long-term Potentiation (LTP)
Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection, which can last for minutes or hours. LTP is based on the Hebbian principle: “cells that fire together wire together. ” There are various mechanisms, none fully understood, behind the synaptic strengthening seen with LTP.
One known mechanism involves a type of postsynaptic glutamate receptor: NMDA (N-Methyl-D-aspartate) receptors. These receptors are normally blocked by magnesium ions. However, when the postsynaptic neuron is depolarized by multiple presynaptic inputs in quick succession (either from one neuron or multiple neurons), the magnesium ions are forced out and Ca 2+ ions pass into the postsynaptic cell. Next, Ca 2+ ions entering the cell initiate a signaling cascade that causes a different type of glutamate receptor, AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, to be inserted into the postsynaptic membrane. Activated AMPA receptors allow positive ions to enter the cell.
Therefore, the next time glutamate is released from the presynaptic membrane, it will have a larger excitatory effect (EPSP) on the postsynaptic cell because the binding of glutamate to these AMPA receptors will allow more positive ions into the cell. The insertion of additional AMPA receptors strengthens the synapse so that the postsynaptic neuron is more likely to fire in response to presynaptic neurotransmitter release. Some drugs co-opt the LTP pathway; this synaptic strengthening can lead to addiction.
Long-term Depression (LTD)
Long-term depression (LTD) is essentially the reverse of LTP: it is a long-term weakening of a synaptic connection. One mechanism known to cause LTD also involves AMPA receptors. In this situation, calcium that enters through NMDA receptors initiates a different signaling cascade, which results in the removal of AMPA receptors from the postsynaptic membrane. With the decrease in AMPA receptors in the membrane, the postsynaptic neuron is less responsive to the glutamate released from the presynaptic neuron. While it may seem counterintuitive, LTD may be just as important for learning and memory as LTP. The weakening and pruning of unused synapses trims unimportant connections, leaving only the salient connections strengthened by long-term potentiation.
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Key Points
- Short-term synaptic enhancement occurs when the amount of available neurotransmitter is increased, while short-term synaptic depression occurs when the amount of vesicles with neurotransmitters is decreased.
- Synapses are strengthened in long-term potentiation (LTP) when AMPA receptors (which bind to negatively-charged glutamate) are increased, allowing more calcium ions to enter the cell, causing a higher excitatory response.
- Long-term depression (LTD) occurs when the AMPA receptors are decreased, which decreases the amount of calcium ions entering the cell, weakening the synaptic response to the release of neurotransmitters.
- The strengthening and weakening of synapses over time controls learning and memory in the brain.
Key Terms
- long-term potentiation : a long-lasting (hours in vitro, weeks to months in vivo) increase, typically in amplitude, of the response of a postsynaptic neuron to a particular pattern of stimuli from a presynaptic neuron
- long-term depression : a long-term weakening of a synaptic connection
- plasticity : the property of neuron that allows it to be strengthened or weakened


6.6: Phase and Group Velocity
Learning Objectives
- Explain phase velocity and group velocity
Phase velocity
A wavefront is a line or surface of constant phase. In a snapshot of a wave at one moment of time, the direction of propagation of the wave is across the wavefronts. The visual situation is different in a spacetime diagram.
In \(1 + 1\) dimensions, figure \(\PageIndex{1}\) (1), suppose that the lines represent the crest of the water waves. The surfer is on top of a crest, riding along with it. His velocity vector \(u\) is in the spacetime direction that lies on top of the wavefront, not across it. Clearly both his motion and the propagation of the wave are to the right, not to the left as we might imagine based on experience with snapshots of waves.
In \(2+1\) dimensions, \(\PageIndex{1}\) (2), the surfer’s velocity is visualized as an arrow lying within a plane of constant phase. Given the wave’s phase information, there is more than one possible arrow of this kind. We could try to resolve the ambiguity by requiring that the arrow’s projection into the \(xy\) plane be perpendicular to the intersection of the wavefronts with that plane, but (with the exception of the case where the wave travels at \(c\), Example \(\PageIndex{1}\)) this prescription gives results that change depending on our frame of reference, and the changes are not describable by a Lorentz transformation of the velocity vector. This shows that in the general case, the phase information of the wave, encoded in the frequency covector \(ω\to \), does not describe the direction of the wave’s propagation through space. At most it tells us the wave’s phase velocity, \(ω/k\), which is not really a velocity. All of these are symptoms of the fact that a velocity is supposed to be a vector, but \(ω\to \) is a covector. The phase velocity lacks physical interest, because it is not the velocity at which any “stuff” moves.
Velocity vector of a light wave, given its phase
We’ve seen that in general, the information about the phase of a wave encoded in \(ω\to \) does not determine its direction of propagation. The exception is a wave, such as a light wave, that propagates at \(c\). Let a world-line of propagation of the wave lie along the vector \(\to v\). In the case of a wave propagating at \(c\), we have \(v^2 = 0\) (so that \(\to v\) can’t have the usual normalization for a velocity vector), and the dispersion relation is simply \(ω^2 = 0\). Since the phase stays constant along a world-line of propagation, \(ω\to v = 0\). We therefore find that \(v\) and \(ω\) are two nonzero, lightlike vectors that are orthogonal to each other. But as shown in problem Q10 in chapter 1, this implies that the two vectors are parallel. Thus if we’re given the covector \(ω\to \), we just have to compute its dual \(\to ω\) to find the direction of propagation.
Group velocity
The phase velocity is not the velocity at which “stuff” is transmitted by the wave. The velocity of the stuff is called the group velocity. To have a meaningully defined group velocity, we need to have a wave that is modulated, because an unmodulated wave is an infinite sine wave that stretches off to infinity, and such an unmodulated wave does not transmit any energy or information. An unmodulated wave has the same frequency covector \(ω\to \) throughout all of spacetime, i.e., the same frequency \(ω\) and wavenumber \(k\). One way of describing a modulated wave is by how \(ω\to \) does change.
But the different components of \(ω\to \) are not free to change in any randomly chosen way. Normally they are constrained by a dispersion relation. For example, surface waves in deep water obey the constraint \(C = 0\), where \(C = ω^4 - α^2k^2\) (figure \(\PageIndex{2}\)) and \(α\) is a constant with units of acceleration, relating to the acceleration of gravity. (Since the water is infinitely deep, there is no other scale that could enter into the constraint.)
Now if a certain bump on the envelope with which the wave is modulated visits spacetime events \(P\) and \(Q\), then whatever frequency and wavelength the wave has near the bump are observed to be the same at \(P\) and \(Q\). In general, \(k\) and \(ω\) are constant along the spacetime displacement of any point on the envelope, so the spacetime displacement \(\to r\) from \(P\) to \(Q\) must satisfy the condition \((∇ω)\to r = 0\).
In addition, \(∇ω\) must be tangent to the surface of constraint \(C = 0\), so that the wave always obeys the constraint. Thus given a point \(ω\to \) in frequency space, the direction of propagation \(r\) must be uniquely determined by the constraint. Suppose \(C\) is a well-behaved function, so that it is approximately a linear function of any small change \(∆ω\), i.e., in \(1 + 1\) dimensions we have
\[\Delta C = \frac{\partial C}{\partial \omega }\Delta \omega + \frac{\partial C}{\partial k}\Delta k\]
In this approximation, \(∆C\) is a linear function that acts on a covector \(∆ω\) and gives back a scalar. In other words, \(∆C\) acts like a vector with components
\[\Delta C = \left ( \frac{\partial C}{\partial \omega }, \frac{\partial C}{\partial k} \right )\]
This vector is parallel to \(r\), so that it points in the wave’s direction of propagation through spacetime, and tells us its group velocity \(\left ( \tfrac{\partial C}{\partial k} \right )/ \left ( \tfrac{\partial C}{\partial \omega } \right )\). In our example of water waves, a calculation shows that the group velocity is \(±α/2ω\), which is half the phase velocity.


1.3: Book III
TELEMACHUS VISITS NESTOR AT PYLOS.
but as the sun was rising from the fair sea 24 into the firmament of heaven to shed light on mortals and immortals, they reached Pylos the city of Neleus. Now the people of Pylos were gathered on the sea shore to offer sacrifice of black bulls to Neptune lord of the Earthquake. There were nine guilds with five hundred men in each, and there were nine bulls to each guild. As they were eating the inward meats 25 and burning the thigh bones [on the embers] in the name of Neptune, Telemachus and his crew arrived, furled their sails, brought their ship to anchor, and went ashore.
Minerva led the way and Telemachus followed her. Presently she said, “Telemachus, you must not be in the least shy or nervous; you have taken this voyage to try and find out where your father is buried and how he came by his end; so go straight up to Nestor that we may see what he has got to tell us. Beg of him to speak the truth, and he will tell no lies, for he is an excellent person.”
“But how, Mentor,” replied Telemachus, “dare I go up to Nestor, and how am I to address him? I have never yet been used to holding long conversations with people, and am ashamed to begin questioning one who is so much older than myself.”
“Some things, Telemachus,” answered Minerva, “will be suggested to you by your own instinct, and heaven will prompt you further; for I am assured that the gods have been with you from the time of your birth until now.”
She then went quickly on, and Telemachus followed in her steps till they reached the place where the guilds of the Pylian people were assembled. There they found Nestor sitting with his sons, while his company round him were busy getting dinner ready, and putting pieces of meat on to the spits 26 while other pieces were cooking. When they saw the strangers they crowded round them, took them by the hand and bade them take their places. Nestor’s son Pisistratus at once offered his hand to each of them, and seated them on some soft sheepskins that were lying on the sands near his father and his brother Thrasymedes. Then he gave them their portions of the inward meats and poured wine for them into a golden cup, handing it to Minerva first, and saluting her at the same time.
“Offer a prayer, sir,” said he, “to King Neptune, for it is his feast that you are joining; when you have duly prayed and made your drink offering, pass the cup to your friend that he may do so also. I doubt not that he too lifts his hands in prayer, for man cannot live without God in the world.
As he spoke he handed her the cup. Minerva thought it very right and proper of him to have given it to herself first; 27 she accordingly began praying heartily to Neptune. “O thou,” she cried, “that encirclest the earth, vouchsafe to grant the prayers of thy servants that call upon thee. More especially we pray thee send down thy grace on Nestor and on his sons; thereafter also make the rest of the Pylian people some handsome return for the goodly hecatomb they are offering you. Lastly, grant Telemachus and myself a happy issue, in respect of the matter that has brought us in our ship to Pylos.”
When she had thus made an end of praying, she handed the cup to Telemachus and he prayed likewise. By and by, when the outer meats were roasted and had been taken off the spits, the carvers gave every man his portion and they all made an excellent dinner.
“Now,” said he, “that our guests have done their dinner, it will be best to ask them who they are. Who, then, sir strangers, are you, and from what port have you sailed? Are you traders? or do you sail the seas as rovers with your hand against every man, and every man’s hand against you?”
Telemachus answered boldly, for Minerva had given him courage to ask about his father and get himself a good name.
“Nestor,” said he, “son of Neleus, honour to the Achaean name, you ask whence we come, and I will tell you. We come from Ithaca under Neritum, 28 and the matter about which I would speak is of private not public import. I seek news of my unhappy father Ulysses, who is said to have sacked the town of Troy in company with yourself. We know what fate befell each one of the other heroes who fought at Troy, but as regards Ulysses heaven has hidden from us the knowledge even that he is dead at all, for no one can certify us in what place he perished, nor say whether he fell in battle on the mainland, or was lost at sea amid the waves of Amphitrite. Therefore I am suppliant at your knees, if haply you may be pleased to tell me of his melancholy end, whether you saw it with your own eyes, or heard it from some other traveller, for he was a man born to trouble. Do not soften things out of any pity for me, but tell me in all plainness exactly what you saw. If my brave father Ulysses ever did you loyal service, either by word or deed, when you Achaeans were harassed among the Trojans, bear it in mind now as in my favour and tell me truly all.”
“My friend,” answered Nestor, “you recall a time of much sorrow to my mind, for the brave Achaeans suffered much both at sea, while privateering under Achilles, and when fighting before the great city of king Priam. Our best men all of them fell there—Ajax, Achilles, Patroclus peer of gods in counsel, and my own dear son Antilochus, a man singularly fleet of foot and in fight valiant. But we suffered much more than this; what mortal tongue indeed could tell the whole story? Though you were to stay here and question me for five years, or even six, I could not tell you all that the Achaeans suffered, and you would turn homeward weary of my tale before it ended. Nine long years did we try every kind of stratagem, but the hand of heaven was against us; during all this time there was no one who could compare with your father in subtlety—if indeed you are his son—I can hardly believe my eyes—and you talk just like him too—no one would say that people of such different ages could speak so much alike. He and I never had any kind of difference from first to last neither in camp nor council, but in singleness of heart and purpose we advised the Argives how all might be ordered for the best.
“When, however, we had sacked the city of Priam, and were setting sail in our ships as heaven had dispersed us, then Jove saw fit to vex the Argives on their homeward voyage; for they had not all been either wise or understanding, and hence many came to a bad end through the displeasure of Jove’s daughter Minerva, who brought about a quarrel between the two sons of Atreus.
“The sons of Atreus called a meeting which was not as it should be, for it was sunset and the Achaeans were heavy with wine. When they explained why they had called the people together, it seemed that Menelaus was for sailing homeward at once, and this displeased Agamemnon, who thought that we should wait till we had offered hecatombs to appease the anger of Minerva. Fool that he was, he might have known that he would not prevail with her, for when the gods have made up their minds they do not change them lightly. So the two stood bandying hard words, whereon the Achaeans sprang to their feet with a cry that rent the air, and were of two minds as to what they should do.
“That night we rested and nursed our anger, for Jove was hatching mischief against us. But in the morning some of us drew our ships into the water and put our goods with our women on board, while the rest, about half in number, stayed behind with Agamemnon. We—the other half—embarked and sailed; and the ships went well, for heaven had smoothed the sea. When we reached Tenedos we offered sacrifices to the gods, for we were longing to get home; cruel Jove, however, did not yet mean that we should do so, and raised a second quarrel in the course of which some among us turned their ships back again, and sailed away under Ulysses to make their peace with Agamemnon; but I, and all the ships that were with me pressed forward, for I saw that mischief was brewing. The son of Tydeus went on also with me, and his crews with him. Later on Menelaus joined us at Lesbos, and found us making up our minds about our course—for we did not know whether to go outside Chios by the island of Psyra, keeping this to our left, or inside Chios, over against the stormy headland of Mimas. So we asked heaven for a sign, and were shown one to the effect that we should be soonest out of danger if we headed our ships across the open sea to Euboea. This we therefore did, and a fair wind sprang up which gave us a quick passage during the night to Geraestus, 29 where we offered many sacrifices to Neptune for having helped us so far on our way. Four days later Diomed and his men stationed their ships in Argos, but I held on for Pylos, and the wind never fell light from the day when heaven first made it fair for me.
“Therefore, my dear young friend, I returned without hearing anything about the others. I know neither who got home safely nor who were lost but, as in duty bound, I will give you without reserve the reports that have reached me since I have been here in my own house. They say the Myrmidons returned home safely under Achilles’ son Neoptolemus; so also did the valiant son of Poias, Philoctetes. Idomeneus, again, lost no men at sea, and all his followers who escaped death in the field got safe home with him to Crete. No matter how far out of the world you live, you will have heard of Agamemnon and the bad end he came to at the hands of Aegisthus—and a fearful reckoning did Aegisthus presently pay. See what a good thing it is for a man to leave a son behind him to do as Orestes did, who killed false Aegisthus the murderer of his noble father. You too, then—for you are a tall smart-looking fellow—show your mettle and make yourself a name in story.”
“Nestor son of Neleus,” answered Telemachus, “honour to the Achaean name, the Achaeans applaud Orestes and his name will live through all time for he has avenged his father nobly. Would that heaven might grant me to do like vengeance on the insolence of the wicked suitors, who are ill treating me and plotting my ruin; but the gods have no such happiness in store for me and for my father, so we must bear it as best we may.”
“My friend,” said Nestor, “now that you remind me, I remember to have heard that your mother has many suitors, who are ill disposed towards you and are making havoc of your estate. Do you submit to this tamely, or are public feeling and the voice of heaven against you? Who knows but what Ulysses may come back after all, and pay these scoundrels in full, either single-handed or with a force of Achaeans behind him? If Minerva were to take as great a liking to you as she did to Ulysses when we were fighting before Troy (for I never yet saw the gods so openly fond of any one as Minerva then was of your father), if she would take as good care of you as she did of him, these wooers would soon some of them forget their wooing.”
Telemachus answered, “I can expect nothing of the kind; it would be far too much to hope for. I dare not let myself think of it. Even though the gods themselves willed it no such good fortune could befall me.”
On this Minerva said, “Telemachus, what are you talking about? Heaven has a long arm if it is minded to save a man; and if it were me, I should not care how much I suffered before getting home, provided I could be safe when I was once there. I would rather this, than get home quickly, and then be killed in my own house as Agamemnon was by the treachery of Aegisthus and his wife. Still, death is certain, and when a man’s hour is come, not even the gods can save him, no matter how fond they are of him.”
“Mentor,” answered Telemachus, “do not let us talk about it any more. There is no chance of my father’s ever coming back; the gods have long since counselled his destruction. There is something else, however, about which I should like to ask Nestor, for he knows much more than any one else does. They say he has reigned for three generations so that it is like talking to an immortal. Tell me, therefore, Nestor, and tell me true; how did Agamemnon come to die in that way? What was Menelaus doing? And how came false Aegisthus to kill so far better a man than himself? Was Menelaus away from Achaean Argos, voyaging elsewhither among mankind, that Aegisthus took heart and killed Agamemnon?”
“I will tell you truly,” answered Nestor, “and indeed you have yourself divined how it all happened. If Menelaus when he got back from Troy had found Aegisthus still alive in his house, there would have been no barrow heaped up for him, not even when he was dead, but he would have been thrown outside the city to dogs and vultures, and not a woman would have mourned him, for he had done a deed of great wickedness; but we were over there, fighting hard at Troy, and Aegisthus, who was taking his ease quietly in the heart of Argos, cajoled Agamemnon’s wife Clytemnestra with incessant flattery.
“At first she would have nothing to do with his wicked scheme, for she was of a good natural disposition; 30 moreover there was a bard with her, to whom Agamemnon had given strict orders on setting out for Troy, that he was to keep guard over his wife; but when heaven had counselled her destruction, Aegisthus carried this bard off to a desert island and left him there for crows and seagulls to batten upon—after which she went willingly enough to the house of Aegisthus. Then he offered many burnt sacrifices to the gods, and decorated many temples with tapestries and gilding, for he had succeeded far beyond his expectations.
“Meanwhile Menelaus and I were on our way home from Troy, on good terms with one another. When we got to Sunium, which is the point of Athens, Apollo with his painless shafts killed Phrontis the steersman of Menelaus’ ship (and never man knew better how to handle a vessel in rough weather) so that he died then and there with the helm in his hand, and Menelaus, though very anxious to press forward, had to wait in order to bury his comrade and give him his due funeral rites. Presently, when he too could put to sea again, and had sailed on as far as the Malean heads, Jove counselled evil against him and made it blow hard till the waves ran mountains high. Here he divided his fleet and took the one half towards Crete where the Cydonians dwell round about the waters of the river Iardanus. There is a high headland hereabouts stretching out into the sea from a place called Gortyn, and all along this part of the coast as far as Phaestus the sea runs high when there is a south wind blowing, but after Phaestus the coast is more protected, for a small headland can make a great shelter. Here this part of the fleet was driven on to the rocks and wrecked; but the crews just managed to save themselves. As for the other five ships, they were taken by winds and seas to Egypt, where Menelaus gathered much gold and substance among people of an alien speech. Meanwhile Aegisthus here at home plotted his evil deed. For seven years after he had killed Agamemnon he ruled in Mycene, and the people were obedient under him, but in the eighth year Orestes came back from Athens to be his bane, and killed the murderer of his father. Then he celebrated the funeral rites of his mother and of false Aegisthus by a banquet to the people of Argos, and on that very day Menelaus came home, 31 with as much treasure as his ships could carry.
“Take my advice then, and do not go travelling about for long so far from home, nor leave your property with such dangerous people in your house; they will eat up everything you have among them, and you will have been on a fool’s errand. Still, I should advise you by all means to go and visit Menelaus, who has lately come off a voyage among such distant peoples as no man could ever hope to get back from, when the winds had once carried him so far out of his reckoning; even birds cannot fly the distance in a twelve-month, so vast and terrible are the seas that they must cross. Go to him, therefore, by sea, and take your own men with you; or if you would rather travel by land you can have a chariot, you can have horses, and here are my sons who can escort you to Lacedaemon where Menelaus lives. Beg of him to speak the truth, and he will tell you no lies, for he is an excellent person.”
As he spoke the sun set and it came on dark, whereon Minerva said, “Sir, all that you have said is well; now, however, order the tongues of the victims to be cut, and mix wine that we may make drink-offerings to Neptune, and the other immortals, and then go to bed, for it is bed time. People should go away early and not keep late hours at a religious festival.”
Thus spoke the daughter of Jove, and they obeyed her saying. Men servants poured water over the hands of the guests, while pages filled the mixing-bowls with wine and water, and handed it round after giving every man his drink offering; then they threw the tongues of the victims into the fire, and stood up to make their drink offerings. When they had made their offerings and had drunk each as much as he was minded, Minerva and Telemachus were for going on board their ship, but Nestor caught them up at once and stayed them.
 Do you think I am so poor and short of clothes, or that I have so few cloaks and as to be unable to find comfortable beds both for myself and for my guests? Let me tell you I have store both of rugs and cloaks, and shall not permit the son of my old friend Ulysses to camp down on the deck of a ship—not while I live—nor yet will my sons after me, but they will keep open house as I have done.”
Then Minerva answered, “Sir, you have spoken well, and it will be much better that Telemachus should do as you have said; he, therefore, shall return with you and sleep at your house, but I must go back to give orders to my crew, and keep them in good heart. I am the only older person among them; the rest are all young men of Telemachus’ own age, who have taken this voyage out of friendship; so I must return to the ship and sleep there. Moreover to-morrow I must go to the Cauconians where I have a large sum of money long owing to me. As for Telemachus, now that he is your guest, send him to Lacedaemon in a chariot, and let one of your sons go with him. Be pleased to also provide him with your best and fleetest horses.”
When she had thus spoken, she flew away in the form of an eagle, and all marvelled as they beheld it. Nestor was astonished, and took Telemachus by the hand.  This can have been none other of those who dwell in heaven than Jove’s redoubtable daughter, the Trito-born, who shewed such favour towards your brave father among the Argives.  In return, I will offer you in sacrifice a broad-browed heifer of a year old, unbroken, and never yet brought by man under the yoke. I will gild her horns, and will offer her up to you in sacrifice.”
Thus did he pray, and Minerva heard his prayer. He then led the way to his own house, followed by his sons and sons in law. When they had got there and had taken their places on the benches and seats, he mixed them a bowl of sweet wine that was eleven years old when the housekeeper took the lid off the jar that held it. As he mixed the wine, he prayed much and made drink offerings to Minerva, daughter of Aegis-bearing Jove. Then, when they had made their drink offerings and had drunk each as much as he was minded, the others went home to bed each in his own abode; but Nestor put Telemachus to sleep in the room that was over the gateway along with Pisistratus, who was the only unmarried son now left him. As for himself, he slept in an inner room of the house, with the queen his wife by his side.
 Here aforetime sat Neleus, peer of gods in counsel, but he was now dead, and had gone to the house of Hades; so Nestor sat in his seat sceptre in hand, as guardian of the public weal. His sons as they left their rooms gathered round him, Echephron, Stratius, Perseus, Aretus, and Thrasymedes; the sixth son was Pisistratus, and when Telemachus joined them they made him sit with them. Nestor then addressed them.
“My sons,” said he, “make haste to do as I shall bid you. I wish first and foremost to propitiate the great goddess Minerva, who manifested herself visibly to me during yesterday’s festivities. Go, then, one or other of you to the plain, tell the stockman to look me out a heifer, and come on here with it at once. Another must go to Telemachus’ ship, and invite all the crew, leaving two men only in charge of the vessel. Some one else will run and fetch Laerceus the goldsmith to gild the horns of the heifer. The rest, stay all of you where you are; tell the maids in the house to prepare an excellent dinner, and to fetch seats, and logs of wood for a burnt offering. Tell them also to bring me some clear spring water.”
 The heifer was brought in from the plain, and Telemachus’s crew came from the ship; the goldsmith brought the anvil, hammer, and tongs, with which he worked his gold, and Minerva herself came to accept the sacrifice. Nestor gave out the gold, and the smith gilded the horns of the heifer that the goddess might have pleasure in their beauty. Then Stratius and Echephron brought her in by the horns; Aretus fetched water from the house in a ewer that had a flower pattern on it, and in his other hand he held a basket of barley meal; sturdy Thrasymedes stood by with a sharp axe, ready to strike the heifer, while Perseus held a bucket. Then Nestor began with washing his hands and sprinkling the barley meal, and he offered many a prayer to Minerva as he threw a lock from the heifer’s head upon the fire.
When they had done praying and sprinkling the barley meal 32 Thrasymedes dealt his blow, and brought the heifer down with a stroke that cut through the tendons at the base of her neck, whereon the daughters and daughters in law of Nestor, and his venerable wife Eurydice (she was eldest daughter to Clymenus) screamed with delight. Then they lifted the heifer’s head from off the ground, and Pisistratus cut her throat. When she had done bleeding and was quite dead, they cut her up. They cut out the thigh bones all in due course, wrapped them round in two layers of fat, and set some pieces of raw meat on the top of them; then Nestor laid them upon the wood fire and poured wine over them, while the young men stood near him with five-pronged spits in their hands. When the thighs were burned and they had tasted the inward meats, they cut the rest of the meat up small, put the pieces on the spits and toasted them over the fire.
Meanwhile lovely Polycaste, Nestor’s youngest daughter, washed Telemachus. When she had washed him and anointed him with oil, she brought him a fair mantle and shirt, 33 and he looked like a god as he came from the bath and took his seat by the side of Nestor. When the outer meats were done they drew them off the spits and sat down to dinner where they were waited upon by some worthy henchmen, who kept pouring them out their wine in cups of gold. As soon as they had had enough to eat and drink Nestor said, “Sons, put Telemachus’s horses to the chariot that he may start at once.”
Thus did he speak, and they did even as he had said, and yoked the fleet horses to the chariot. The housekeeper packed them up a provision of bread, wine, and sweet meats fit for the sons of princes. Then Telemachus got into the chariot, while Pisistratus gathered up the reins and took his seat beside him. He lashed the horses on and they flew forward nothing loth into the open country, leaving the high citadel of Pylos behind them. All that day did they travel, swaying the yoke upon their necks till the sun went down and darkness was over all the land. Then they reached Pherae where Diocles lived, who was son to Ortilochus and grandson to Alpheus. Here they passed the night and Diocles entertained them hospitably. When the child of morning, rosy-fingered Dawn, appeared, they again yoked their horses and drove out through the gateway under the echoing gatehouse. 34 Pisistratus lashed the horses on and they flew forward nothing loth; presently they came to the corn lands of the open country, and in the course of time completed their journey, so well did their steeds take them. 35
Now when the sun had set and darkness was over the land,


21.5: Western Canada During the Cenozoic
Two additional relatively small terranes collided with North America early in the Cenozoic. At around 55 Ma metamorphosed sedimentary and volcanic rocks of the Pacific Rim Terrane were forced a few tens of kilometers underneath the west coast of Vancouver Island (Figure \(\PageIndex{1}\)).
These rocks are distributed along the west coast of the island and in the area around Victoria (Figure \(\PageIndex{2}\)). At around 42 Ma, sea-floor pillow basalt and gabbro of the Crescent Terrane was accreted to the southern margin of Vancouver Island and also to the adjacent part of Washington State (Figuer 21.30). These two terranes are shown as “Outboard terranes” in Figure \(\PageIndex{2}\).
The accretion of the Pacific Rim and Crescent Terranes had the effect of pushing Vancouver Island closer to the North American mainland, resulting in the uplift of the sediments deposited within the Nanaimo Basin to form islands in the Strait of Georgia (Figure \(\PageIndex{3}\)) and mountains on Vancouver Island.
Following these events, the subduction of the Juan de Fuca Plate, which is a remnant of the former, much larger, Farallon Plate, was re-established at its current location farther to the west of Vancouver Island. This subduction—and that of the North America Plate beneath Alaska—has produced recently active volcanoes in Alaska, and all along the west coast from southwestern B.C. to northern California (Figure \(\PageIndex{4}\)). In southwestern B.C. there are several dormant volcanoes of Pleistocene age (including Garibaldi and Meager) that trend along a line that also passes through Mt. Baker in Washington State. About 40 km to the east is a trend of slightly older igneous complexes (Pliocene to Oligocene). The displacement between these belts could be explained by a westward shift in the position of the subduction zone over that time period. The subduction and transform boundaries along this coast also generate relatively frequent earthquakes throughout this region, as illustrated in Exercise 11.1.
Sedimentation in the Western Canada Sedimentary Basin continued into the Cenozoic (Figure \(\PageIndex{10}\)) with deposition of the Paskapoo Formation adjacent to the Rockies in Alberta (Figure \(\PageIndex{6}\)), the Ravenscrag Formation in the Cypress Hills of southern Alberta and Saskatchewan, and the Turtle Hills Formation in southern Manitoba. All of these strata were deposited in terrestrial fluvial and deltaic environments, and all of them include coal deposits. Numerous mammalian (and other) fossils have been found in these rocks in Alberta and Saskatchewan. The mammals include primitive ungulates (ancestors to the deer and their relatives), a type of pangolin, a colugo (a gliding mammal that was possibly a primate ancestor), and some true primates in the suborder Plesiadapiformes, which became extinct and are not ancestors to any modern primates.
The area underlain by the Paleocene Paskapoo Formation is outlined in yellow on the map shown here. The Paskapoo ranges up to about 1,000 m thick, and contours of its thickness (known as isopachs) are shown. The average thickness is about 500 m, and the area covered by the formation is about 90,000 km 2 . This means that the rock has a volume of about 45,000 km 3 . The sediments of the Paskapoo were derived from the Paleocene Rocky Mountains, within the area shown in blue, which is about 60,000 km 2 .
- What average depth of erosion would have been required within the source area to produce the 45,000 km 3 of sediment, assuming that all of the eroded sediment ended up in the Paskapoo Formation?
- The Paskapoo was deposited over a period of 4 million years from 62.5 Ma to 58.5 Ma. Assuming an average thickness of 500 m, what was the average rate of deposition (in mm/year) over that period?
See Appendix 3 for Exercise 21.5 answers.
Rocks younger than Paleocene (i.e., younger than 55 Ma) are relatively rare across the prairies, but there are widespread Eocene-aged volcanic and sedimentary rocks in central and southern B.C. The Kamloops Group includes the Tranquille Formation, made up of lacustrine (lake deposited) sediments, overlain by the Dewdrop Flats Formation of basaltic and andesitic volcanic flows and breccias. The Tranquille Formation includes the McAbee Beds and a number of other important sites with Eocene fossils (Figure \(\PageIndex{8}\)).
The earliest Pleistocene glaciation in Canada started at about 2.64 Ma (late Pliocene) in the Klondike area of Yukon. This was part of the Cordilleran Ice Sheet. The Laurentide Ice Sheet started to form shortly afterward, and within 200,000 years had covered a large part of Canada and extended well into the United States. The Pleistocene glaciations had a major impact on the topography and geology of western Canada, creating extraordinary glacial erosion features in the mountainous regions of the west (Figure \(\PageIndex{9}\)), and leaving enormous volumes of glacial sediment and glacial depositional features throughout the region (Figure \(\PageIndex{10}\)).
Image Descriptions
Figure \(\PageIndex{4}\) image description: The Juan de Fuca Plate lies between the Pacific and the North America plates along the south coast of British Columbia and the northwest coast of the United States. It is subducting beneath the North America Plate to form mountains along the west coast of North America. The Pacific Plate and the Juan de Fuca Plate are pulling apart to create divergent and transform boundaries. The Pacific Plate and the North America Plate have a transform boundary along the north coast of British Columbia, but the Pacific Plate is subducting beneath the North America Plate along the Alaskan coast. [Return to Figure \(\PageIndex{4}\)]
Media Attributions
- Figure \(\PageIndex{1}\): © Steven Earle. CC BY. After the Geological Survey of Canada.
- Figure \(\PageIndex{2}\): © Steven Earle. CC BY. After the Geological Survey of Canada.
- Figure \(\PageIndex{3}\), 21.5.4, 21.5.5: © Steven Earle. CC BY.
- Figure \(\PageIndex{6}\): “Paskapoo Mudstones Red Deer” by Georgialh. CC BY-SA.
- Figure \(\PageIndex{7}\): © Steven Earle. CC BY. After Alberta Geological Survey.
- FIgure 21.5.8: © Steven Earle. CC BY.
- Figure \(\PageIndex{9}\): © Isaac Earle. CC BY.
- Figure \(\PageIndex{10}\): A NASA Landsat. © Google Earth. Used with permission for non-commerical purposes.


2.5: Reading- Absolute Advantage
GOALS
By the end of this section, you will be able to:
- Discuss globalization of markets, economies, and jobs.
- Explain international trade, foreign direct investments, and global monetary systems.
Absolute advantage and balance of trade are two important aspects of international trade that affect countries and organizations.
KEY Points
- Absolute advantage: In economics, the principle of absolute advantage refers to the ability of a party (an individual, or firm, or country) to produce more of a good or service than competitors, using the same amount of resources.
- Net exports: The balance of trade (or net exports, sometimes symbolized as NX) is the difference between the monetary value of exports and imports of output in an economy over a certain period. It is the relationship between a nation’s imports and exports.
- Advantageous trade: Advantageous trade is based on comparative advantage and covers a larger set of circumstances while still including the case of absolute advantage and hence is a more general theory.
Terms
- Absolute advantage : The capability to produce more of a given product using less of a given resource than a competing entity.
- Advantageous : Being of advantage; conferring advantage; gainful; profitable; useful; beneficial; as, an advantageous position.
In the drive for international trade, it is important to understand how trade affects countries positively and negatively—both how a country’s imports and exports affect its economy and how effectively the country’s ability to create and exportvital goods effects the businesses within that country. Absolute advantage and balance of trade are two important aspects of international trade that affect countries and organizations .
Absolute Advantage
In economics, the principle of absolute advantage refers to the ability of a party (an individual, a firm, or a country) to produce more of a good or service than competitors while using the same amount of resources. Adam Smith first described the principle of absolute advantage in the context of international trade, using labor as the only input. Since absolute advantage is determined by a simple comparison of labor productivities, it is possible for a party to have no absolute advantage in anything; in that case, according to the theory of absolute advantage, no trade will occur with the other party. It can be contrasted with the concept of comparative advantage, which refers to the ability to produce a particular good at a lower opportunity cost.
Balance of Trade
The balance of trade (or net exports, sometimes symbolized as NX) is the difference between the monetary value of exports and imports in an economy over a certain period. A positive balance is known as a trade surplus if it consists ofexporting more than is imported; a negative balance is referred to as a trade deficit or, informally, a trade gap. The balance of trade is sometimes divided into a goods and a services balance.
GLOSSARY
Balance of trade
The difference between the monetary value of exports and imports in an economy over a certain period of time.
Comparative advantage
The ability of a party to produce a particular good or service at a lower marginal and opportunity cost over another. The concept that a certain good can be produced more efficiently than others due to a number of factors, including productive skills, climate, natural resource availability, and so forth.
Economy
Collective focus of the study of money, currency and trade, and the efficient use of resources.The system of production and distribution and consumption. The overall measure of a currency system; as the national economy.
Export
To sell (goods) to a foreign country. Any good or commodity, transported from one country to another country in a legitimate fashion, typically for use in trade.
Exporting
The act of selling to a foreign country the sale of capital, goods, and services across international borders or territories.
Good
An object produced for market.
Input
Something fed into a process with the intention of it shaping or affecting the outputs of that process. Each participant’s contributions that are viewed as entitling him/her to rewards or costs. Examples include time, effort, and loyalty.
Opportunity cost
The cost of an opportunity forgone (and the loss of the benefits that could be received from that opportunity); the most valuable forgone alternative. The cost of any activity measured in terms of the value of the next best alternative forgone (that is not chosen). The value forfeited by taking a particular route.
Output
Production; quantity produced, created, or completed. data sent out of the computer, as to output device such as a monitor or printer.
Productivity
Productivity is a measure of the efficiency of production and is defined as total output per one unit of a total input. The rate at which goods or services are produced by a standard population of workers. A ratio of production output to what is required to produce it (inputs). The state of being productive, fertile, or efficient. The rate at which products and services are produced relative to a particular workforce.
Resource
Something that one uses to achieve an objective, e.g. raw materials or personnel.
Services
That which is produced, then traded, bought or sold, then finally consumed and consists of an action or work.
Trade deficit
A negative balance of trade.
Trade surplus
A positive balance of trade.
Value
The degree of importance given to something.  A customer’s perception of relative price (the cost to own and use) and performance (quality)
Values
A collection of guiding principles; what one deems to be correct and desirable in life, especially regarding personal conduct.
- Boundless Business. Authored by : Boundless. Provided by : Boundless. Located at : https://www.boundless.com/business/ . License : CC BY-SA: Attribution-ShareAlike
- Image of EFTA logo. Authored by : EFTA. Located at : http://commons.wikimedia.org/wiki/File:EFTA_logo.svg . License : Public Domain: No Known Copyright


26.1: Body Fluids and Fluid Compartment
Learning Objectives
- Explain the importance of water in the body
- Contrast the composition of the intracellular fluid with that of the extracellular fluid
- Explain the importance of protein channels in the movement of solutes
- Identify the causes and symptoms of edema
The chemical reactions of life take place in aqueous solutions. The dissolved substances in a solution are called solutes. In the human body, solutes vary in different parts of the body, but may include proteins—including those that transport lipids, carbohydrates, and, very importantly, electrolytes. Often in medicine, a mineral dissociated from a salt that carries an electrical charge (an ion) is called and electrolyte. For instance, sodium ions (Na + ) and chloride ions (Cl - ) are often referred to as electrolytes.
In the body, water moves through semi-permeable membranes of cells and from one compartment of the body to another by a process called osmosis. Osmosis is basically the diffusion of water from regions of higher concentration to regions of lower concentration, along an osmotic gradient across a semi-permeable membrane. As a result, water will move into and out of cells and tissues, depending on the relative concentrations of the water and solutes found there. An appropriate balance of solutes inside and outside of cells must be maintained to ensure normal function.
Body Water Content
Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 50–60 percent in adult men and women, to as low as 45 percent in old age. The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone, and other tissues change from infancy to adulthood (Figure \(\PageIndex{1}\)).  In contrast, teeth have the lowest proportion of water, at 8–10 percent.
Fluid Compartments
Body fluids can be discussed in terms of their specific fluid compartment , a location that is largely separate from another compartment by some form of a physical barrier. The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. Extracellular fluid (ECF) surrounds all cells in the body. Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood (Figure \(\PageIndex{2}\)).
Intracellular Fluid
The ICF lies within cells and is the principal component of the cytosol/cytoplasm. The ICF makes up about 60 percent of the total water in the human body, and in an average-size adult male, the ICF accounts for about 25 liters (seven gallons) of fluid (Figure \(\PageIndex{3}\)). This fluid volume tends to be very stable, because the amount of water in living cells is closely regulated. If the amount of water inside a cell falls to a value that is too low, the cytosol becomes too concentrated with solutes to carry on normal cellular activities; if too much water enters a cell, the cell may burst and be destroyed.
Extracellular Fluid
The ECF accounts for the other one-third of the body’s water content. Approximately 20 percent of the ECF is found in plasma. Plasma travels through the body in blood vessels and transports a range of materials, including blood cells, proteins (including clotting factors and antibodies), electrolytes, nutrients, gases, and wastes. Gases, nutrients, and waste materials travel between capillaries and cells through the IF. Cells are separated from the IF by a selectively permeable cell membrane that helps regulate the passage of materials between the IF and the interior of the cell.
The body has other water-based ECF. These include the cerebrospinal fluid that bathes the brain and spinal cord, lymph, the synovial fluid in joints, the pleural fluid in the pleural cavities, the pericardial fluid in the cardiac sac, the peritoneal fluid in the peritoneal cavity, and the aqueous humor of the eye. Because these fluids are outside of cells, these fluids are also considered components of the ECF compartment.
Composition of Body Fluids
The compositions of the two components of the ECF—plasma and IF—are more similar to each other than either is to the ICF (Figure \(\PageIndex{4}\)). Blood plasma has high concentrations of sodium, chloride, bicarbonate, and protein. The IF has high concentrations of sodium, chloride, and bicarbonate, but a relatively lower concentration of protein. In contrast, the ICF has elevated amounts of potassium, phosphate, magnesium, and protein. Overall, the ICF contains high concentrations of potassium and phosphate (HPO 4 2- ), whereas both plasma and the ECF contain high concentrations of sodium and chloride.
Most body fluids are neutral in charge. Thus, cations, or positively charged ions, and anions, or negatively charged ions, are balanced in fluids. As seen in the previous graph, sodium (Na + ) ions and chloride (Cl - ) ions are concentrated in the ECF of the body, whereas potassium (K + ) ions are concentrated inside cells. Although sodium and potassium can “leak” through “pores” into and out of cells, respectively, the high levels of potassium and low levels of sodium in the ICF are maintained by sodium-potassium pumps in the cell membranes. These pumps use the energy supplied by ATP to pump sodium out of the cell and potassium into the cell (Figure \(\PageIndex{5}\)).
Fluid Movement between Compartments
Hydrostatic pressure , the force exerted by a fluid against a wall, causes movement of fluid between compartments. The hydrostatic pressure of blood is the pressure exerted by blood against the walls of the blood vessels by the pumping action of the heart. In capillaries, hydrostatic pressure (also known as capillary blood pressure) is higher than the opposing “colloid osmotic pressure” in blood—a “constant” pressure primarily produced by circulating albumin—at the arteriolar end of the capillary (Figure \(\PageIndex{6}\)). This pressure forces plasma and nutrients out of the capillaries and into surrounding tissues. Fluid and the cellular wastes in the tissues enter the capillaries at the venule end, where the hydrostatic pressure is less than the osmotic pressure in the vessel. Filtration pressure squeezes fluid from the plasma in the blood to the IF surrounding the tissue cells. The surplus fluid in the interstitial space that is not returned directly back to the capillaries is drained from tissues by the lymphatic system, and then re-enters the vascular system at the subclavian veins.
Fluid Balance: Edema
Edema is the accumulation of excess water in the tissues. It is most common in the soft tissues of the extremities. The physiological causes of edema include water leakage from blood capillaries. Edema is almost always caused by an underlying medical condition, by the use of certain therapeutic drugs, by pregnancy, by localized injury, or by an allergic reaction. In the limbs, the symptoms of edema include swelling of the subcutaneous tissues, an increase in the normal size of the limb, and stretched, tight skin. One quick way to check for subcutaneous edema localized in a limb is to press a finger into the suspected area. Edema is likely if the depression persists for several seconds after the finger is removed (which is called “pitting”).
Pulmonary edema is excess fluid in the air sacs of the lungs, a common symptom of heart and/or kidney failure. People with pulmonary edema likely will experience difficulty breathing, and they may experience chest pain. Pulmonary edema can be life threatening, because it compromises gas exchange in the lungs, and anyone having symptoms should immediately seek medical care.
In pulmonary edema resulting from heart failure, excessive leakage of water occurs because fluids get “backed up” in the pulmonary capillaries of the lungs, when the left ventricle of the heart is unable to pump sufficient blood into the systemic circulation. Because the left side of the heart is unable to pump out its normal volume of blood, the blood in the pulmonary circulation gets “backed up,” starting with the left atrium, then into the pulmonary veins, and then into pulmonary capillaries. The resulting increased hydrostatic pressure within pulmonary capillaries, as blood is still coming in from the pulmonary arteries, causes fluid to be pushed out of them and into lung tissues.
Other causes of edema include damage to blood vessels and/or lymphatic vessels, or a decrease in osmotic pressure in chronic and severe liver disease, where the liver is unable to manufacture plasma proteins (Figure \(\PageIndex{8}\)). A decrease in the normal levels of plasma proteins results in a decrease of colloid osmotic pressure (which counterbalances the hydrostatic pressure) in the capillaries. This process causes loss of water from the blood to the surrounding tissues, resulting in edema.
Mild, transient edema of the feet and legs may be caused by sitting or standing in the same position for long periods of time, as in the work of a toll collector or a supermarket cashier. This is because deep veins in the lower limbs rely on skeletal muscle contractions to push on the veins and thus “pump” blood back to the heart. Otherwise, the venous blood pools in the lower limbs and can leak into surrounding tissues.
Medications that can result in edema include vasodilators, calcium channel blockers used to treat hypertension, non-steroidal anti-inflammatory drugs, estrogen therapies, and some diabetes medications. Underlying medical conditions that can contribute to edema include congestive heart failure, kidney damage and kidney disease, disorders that affect the veins of the legs, and cirrhosis and other liver disorders.
Therapy for edema usually focuses on elimination of the cause. Activities that can reduce the effects of the condition include appropriate exercises to keep the blood and lymph flowing through the affected areas. Other therapies include elevation of the affected part to assist drainage, massage and compression of the areas to move the fluid out of the tissues, and decreased salt intake to decrease sodium and water retention.
Chapter Review
Your body is mostly water. Body fluids are aqueous solutions with differing concentrations of materials, called solutes. An appropriate balance of water and solute concentrations must be maintained to ensure cellular functions. If the cytosol becomes too concentrated due to water loss, cell functions deteriorate. If the cytosol becomes too dilute due to water intake by cells, cell membranes can be damaged, and the cell can burst. Hydrostatic pressure is the force exerted by a fluid against a wall and causes movement of fluid between compartments. Fluid can also move between compartments along an osmotic gradient. Active transport processes require ATP to move some solutes against their concentration gradients between compartments. Passive transport of a molecule or ion depends on its ability to pass easily through the membrane, as well as the existence of a high to low concentration gradient.
Interactive Link Questions
Q. Watch this video to learn more about body fluids, fluid compartments, and electrolytes. When blood volume decreases due to sweating, from what source is water taken in by the blood?
Answer: The interstitial fluid (IF).
Q. Watch this video to see an explanation of the dynamics of fluid in the body’s compartments. What happens in tissues when capillary blood pressure is less than osmotic pressure?
Answer: Fluid enters the capillaries from interstitial spaces.
Review Questions
Q. Solute contributes to the movement of water between cells and the surrounding medium by ________.
A. osmotic pressure
B. hydrostatic pressure
C. Brownian movement
D. random motion
Answer: A
Q. A cation has a(n) ________ charge.
A. neutral
B. positive
C. alternating
D. negative
Answer: B
Q. Interstitial fluid (IF) is ________.
A. the fluid in the cytosol of the cells
B. the fluid component of blood
C. the fluid that bathes all of the body’s cells except for blood cells
D. the intracellular fluids found between membranes
Answer: C
Critical Thinking Questions
Q. Plasma contains more sodium than chloride. How can this be if individual ions of sodium and chloride exactly balance each other out, and plasma is electrically neutral?
A. There are additional negatively charged molecules in plasma besides chloride. The additional sodium balances the total negative charges.
Q. How is fluid moved from compartment to compartment?
A. Fluid is moved by a combination of osmotic and hydrostatic pressures. The osmotic pressure results from differences in solute concentrations across cell membranes. Hydrostatic pressure results from the pressure of blood as it enters a capillary system, forcing some fluid out of the vessel into the surrounding tissues.
Glossary
- extracellular fluid (ECF)
- fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluids found in other reservoirs in the body
- fluid compartment
- fluid inside all cells of the body constitutes a compartment system that is largely segregated from other systems
- hydrostatic pressure
- pressure exerted by a fluid against a wall, caused by its own weight or pumping force
- interstitial fluid (IF)
- fluid in the small spaces between cells not contained within blood vessels
- intracellular fluid (ICF)
- fluid in the cytosol of cells


4.1: Introduction to Electric Potential and Electric Energy
Two of the most familiar aspects of electricity are its energy and voltage . We know, for example, that great amounts of electrical energy can be stored in batteries, are transmitted cross-country through power lines, and may jump from clouds to explode the sap of trees. In a similar manner, at molecular levels, ions cross cell membranes and transfer information. We also know about voltages associated with electricity. Batteries are typically a few volts, the outlets in your home produce 120 volts, and power lines can be as high as hundreds of thousands of volts.
Energy and voltage are not the same thing. A motorcycle battery, for example, is small and would not be very successful in replacing the much larger car battery, yet each has the same voltage. In this chapter, we shall examine the relationship between voltage and electrical energy.


3.1: Organic Molecules
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- Ying Liu
- City College of San Francisco
Learning Objectives
- Identify common elements and structures found in organic molecules
- Distinguish between macronutrients and micronutrients (trace elements)
- Differentiate inorganic from organic molecules
Biochemistry is the discipline that studies the chemistry of life, and its objective is to explain form and function based on chemical principles. Organic chemistry is the discipline devoted to the study of carbon-based chemistry, which is the foundation for the study of biomolecules and the discipline of biochemistry. Both biochemistry and organic chemistry are based on the concepts of general chemistry, some of which are presented in Appendix A .
Elements in Living Cells
The most abundant element in cells is hydrogen (H), followed by carbon (C), oxygen (O), nitrogen (N), phosphorous (P), and sulfur (S). We call these elements macronutrients , and they account for about 99% of the dry weight of cells. Some elements, such as sodium (Na), potassium (K), magnesium (Mg), zinc (Zn), iron (Fe), calcium (Ca), molybdenum (Mo), copper (Cu), cobalt (Co), manganese (Mn), or vanadium (Va), are required by some cells in very small amounts and are called micronutrients or trace elements . All of these elements are essential to the function of many biochemical reactions, and, therefore, are essential to life.
The four most abundant elements in living matter (C, N, O, and H) have low atomic numbers and are thus light elements capable of forming strong bonds with other atoms to produce molecules (Figure \(\PageIndex{1}\)). Carbon forms four chemical bonds, whereas nitrogen forms three, oxygen forms two, and hydrogen forms one. When bonded together within molecules, oxygen, sulfur, and nitrogen often have one or more “lone pairs” of electrons that play important roles in determining many of the molecules’ physical and chemical properties (see Appendix A ). These traits in combination permit the formation of a vast number of diverse molecular species necessary to form the structures and enable the functions of living organisms.
Living organisms contain inorganic compounds (mainly water and salts; see Appendix A ) and organic molecules. Organic molecules contain carbon; inorganic compounds do not. Carbon oxides and carbonates are exceptions; they contain carbon but are considered inorganic because they do not contain hydrogen. The atoms of an organic molecule are typically organized around chains of carbon atoms.
Inorganic compounds make up 1%–1.5% of a living cell’s mass. They are small, simple compounds that play important roles in the cell, although they do not form cell structures. Most of the carbon found in organic molecules originates from inorganic carbon sources such as carbon dioxide captured via carbon fixation by microorganisms.
Query \(\PageIndex{1}\)
Query \(\PageIndex{1}\)
Exercise \(\PageIndex{2}\)
- Describe the most abundant elements in nature.
- What are the differences between organic and inorganic molecules?
Key Concepts and Summary
- The most abundant elements in cells are hydrogen, carbon, oxygen, nitrogen, phosphorus, and sulfur.


6: Conclusion
For important issues, we need well-developed reasons or arguments to decide what to believe and do about the issues. Many people say they just “feel” that abortion is wrong or their “opinion” is that it’s not wrong. But complex issues require informed, fair and honest critical thinking, not just mere “feelings” or “opinions,” and we hope this essay has modeled this type of systematic and serious engagement with the arguments and evidence. We hope that readers’ reflective observations about how we have stated and evaluated arguments will help them improve their own skills at engaging arguments on this and other issues, on their own and in discussion with others.
We have focused on disagreements about abortion, but we want to end on an agreement. Everyone agrees there should be fewer abortions. Even people who believe abortions are generally not wrong don’t think that having an abortion is just a great way to spend time and resources. So everyone could agree that we, as a society, should do more to reduce the “demand” for abortions: we could address the many causes that lead women to seek abortions. 19 Some other countries don’t have as many abortions as the US does. In many cases this is because of deliberate choices they have made to make their countries more supportive of all of their citizens and make it easier for them to meet their economic, medical and familial needs. We too could be Good Samaritans, in these ways and more. This would be very good, not just for this issue but for who we are as people.


<IP_ADDRESS>: Crossing Brooklyn Ferry
Flood-tide below me! I see you face to face!
Clouds of the west—sun there half an hour high—I see you also face to face.
Crowds of men and women attired in the usual costumes, how curious you are to me!
On the ferry-boats the hundreds and hundreds that cross, returning home, are more curious to me than you suppose,
And you that shall cross from shore to shore years hence are more to me, and more in my meditations, than you might suppose.
2The impalpable sustenance of me from all things at all hours of the day,
The simple, compact, well-join'd scheme, myself disintegrated, every one disintegrated yet part of the scheme,
The similitudes of the past and those of the future,
The glories strung like beads on my smallest sights and hearings, on the walk in the street and the passage over the river,
The current rushing so swiftly and swimming with me far away,
The others that are to follow me, the ties between me and them,
The certainty of others, the life, love, sight, hearing of others.
Others will enter the gates of the ferry and cross from shore to shore,
Others will watch the run of the flood-tide,
Others will see the shipping of Manhattan north and west, and the heights of Brooklyn to the south and east,
Others will see the islands large and small;
Fifty years hence, others will see them as they cross, the sun half an hour high,
A hundred years hence, or ever so many hundred years hence, others will see them,
Will enjoy the sunset, the pouring-in of the flood-tide, the falling-back to the sea of the ebb-tide.
3It avails not, time nor place—distance avails not,
I am with you, you men and women of a generation, or ever so many generations hence,
Just as you feel when you look on the river and sky, so I felt,
Just as any of you is one of a living crowd, I was one of a crowd,
Just as you are refresh'd by the gladness of the river and the bright flow, I was refresh'd,
Just as you stand and lean on the rail, yet hurry with the swift current, I stood yet was hurried,
Just as you look on the numberless masts of ships and the thick-stemm'd pipes of steamboats, I look'd.
I too many and many a time cross'd the river of old,
Watched the Twelfth-month sea-gulls, saw them high in the air floating with motionless wings, oscillating their bodies,
Saw how the glistening yellow lit up parts of their bodies and left the rest in strong shadow,
Saw the slow-wheeling circles and the gradual edging toward the south,
Saw the reflection of the summer sky in the water,
Had my eyes dazzled by the shimmering track of beams,
Look'd at the fine centrifugal spokes of light round the shape of my head in the sunlit water,
Look'd on the haze on the hills southward and south-westward,
Look'd on the vapor as it flew in fleeces tinged with violet,
Look'd toward the lower bay to notice the vessels arriving,
Saw their approach, saw aboard those that were near me,
Saw the white sails of schooners and sloops, saw the ships at anchor,
The sailors at work in the rigging or out astride the spars,
The round masts, the swinging motion of the hulls, the slender serpentine pennants,
The large and small steamers in motion, the pilots in their pilot-houses,
The white wake left by the passage, the quick tremulous whirl of the wheels,
The flags of all nations, the falling of them at sunset,
The scallop-edged waves in the twilight, the ladled cups, the frolicsome crests and glistening,
The stretch afar growing dimmer and dimmer, the gray walls of the granite storehouses by the docks,
On the river the shadowy group, the big steam-tug closely flank'd on each side by the barges, the hay-boat, the belated lighter,
On the neighboring shore the fires from the foundry chimneys burning high and glaringly into the night,
Casting their flicker of black contrasted with wild red and yellow light over the tops of houses, and down into the clefts of streets.
4These and all else were to me the same as they are to you,
I loved well those cities, loved well the stately and rapid river,
The men and women I saw were all near to me,
Others the same—others who look back on me because I look'd forward to them,
(The time will come, though I stop here to-day and to-night.)
5What is it then between us?
What is the count of the scores or hundreds of years between us?
Whatever it is, it avails not—distance avails not, and place avails not,
I too lived, Brooklyn of ample hills was mine,
I too walk'd the streets of Manhattan island, and bathed in the waters around it,
I too felt the curious abrupt questionings stir within me,
In the day among crowds of people sometimes they came upon me,
In my walks home late at night or as I lay in my bed they came upon me,
I too had been struck from the float forever held in solution,
I too had receiv'd identity by my body,
That I was I knew was of my body, and what I should be I knew I should be of my body.
6It is not upon you alone the dark patches fall,
The dark threw its patches down upon me also,
The best I had done seem'd to me blank and suspicious,
My great thoughts as I supposed them, were they not in reality meagre?
Nor is it you alone who know what it is to be evil,
I am he who knew what it was to be evil,
I too knitted the old knot of contrariety,
Blabb'd, blush'd, resented, lied, stole, grudg'd,
Had guile, anger, lust, hot wishes I dared not speak,
Was wayward, vain, greedy, shallow, sly, cowardly, malignant,
The wolf, the snake, the hog, not wanting in me,
The cheating look, the frivolous word, the adulterous wish, not wanting,
Refusals, hates, postponements, meanness, laziness, none of these wanting,
Was one with the rest, the days and haps of the rest,
Was call'd by my nighest name by clear loud voices of young men as they saw me approaching or passing,
Felt their arms on my neck as I stood, or the negligent leaning of their flesh against me as I sat,
Saw many I loved in the street or ferry-boat or public assembly, yet never told them a word,
Lived the same life with the rest, the same old laughing, gnawing, sleeping,
Play'd the part that still looks back on the actor or actress,
The same old role, the role that is what we make it, as great as we like,
Or as small as we like, or both great and small.
7Closer yet I approach you,
What thought you have of me now, I had as much of you—I laid in my stores in advance,
I consider'd long and seriously of you before you were born.
Who was to know what should come home to me?
Who knows but I am enjoying this?
Who knows, for all the distance, but I am as good as looking at you now, for all you cannot see me?
8Ah, what can ever be more stately and admirable to me than mast-hemm'd Manhattan?
River and sunset and scallop-edg'd waves of flood-tide?
The sea-gulls oscillating their bodies, the hay-boat in the twilight, and the belated lighter?
What gods can exceed these that clasp me by the hand, and with voices I love call me promptly and loudly by my nighest name as I approach?
What is more subtle than this which ties me to the woman or man that looks in my face?
Which fuses me into you now, and pours my meaning into you?
We understand then do we not?
What I promis'd without mentioning it, have you not accepted?
What the study could not teach—what the preaching could not accomplish is accomplish'd, is it not?
9Flow on, river! flow with the flood-tide, and ebb with the ebb-tide!
Frolic on, crested and scallop-edg'd waves!
Gorgeous clouds of the sunset! drench with your splendor me, or the men and women generations after me!
Cross from shore to shore, countless crowds of passengers!
Stand up, tall masts of Mannahatta! stand up, beautiful hills of Brooklyn!
Throb, baffled and curious brain! throw out questions and answers!
Suspend here and everywhere, eternal float of solution!
Gaze, loving and thirsting eyes, in the house or street or public assembly!
Sound out, voices of young men! loudly and musically call me by my nighest name!
Live, old life! play the part that looks back on the actor or actress!
Play the old role, the role that is great or small according as one makes it!
Consider, you who peruse me, whether I may not in unknown ways be looking upon you;
Be firm, rail over the river, to support those who lean idly, yet haste with the hasting current;
Fly on, sea-birds! fly sideways, or wheel in large circles high in the air;
Receive the summer sky, you water, and faithfully hold it till all downcast eyes have time to take it from you!
Diverge, fine spokes of light, from the shape of my head, or any one's head, in the sunlit water!
Come on, ships from the lower bay! pass up or down, white-sail'd schooners, sloops, lighters!
Flaunt away, flags of all nations! be duly lower'd at sunset!
Burn high your fires, foundry chimneys! cast black shadows at nightfall! cast red and yellow light over the tops of the houses!
Appearances, now or henceforth, indicate what you are,
You necessary film, continue to envelop the soul,
About my body for me, and your body for you, be hung out divinest aromas,
Thrive, cities—bring your freight, bring your shows, ample and sufficient rivers,
Expand, being than which none else is perhaps more spiritual,
Keep your places, objects than which none else is more lasting.